In the oil and gas exploration industry, the process of cementing casing into the wellbore of an oil or gas well generally comprises several steps. For example, a section of a hole or wellbore is drilled with a drill bit which is slightly larger than the outside diameter of the casing which will be run into the well. Next, after drilling the well to the desired depth, the drillpipe is removed and a string of casing is run into the wellbore to the required depth where the casing lands in and is supported by a well head. When run in, the casing string is typically supported by the derrick of the drilling rig used to drill the wellbore. The casing string typically has a bottom assembly attached to it, such as a guide shoe or a float shoe, that guides the casing string into the borehole. At this time, the drilling mud (used to remove formation cuttings during the drilling of the well) is still in the borehole. For the casing to be cemented in place, this mud must be removed and replaced by hardened cement.
For the cementing operations, cement slurry is pumped into the casing to fill the annulus between the casing and the wellbore. The cement passes out of apertures in the shoe and into the annulus between the borehole and the casing. The drilling mud is displaced upwards and the cement replaces it in the annulus. The cement needs to extend at least as far up the annulus so as to cover the production and/or water zones, and the previous casing shoe if present, and sometimes the cement even extends to the surface. The cement serves to secure the casing in position and prevent migration of fluids and gasses between formations through which the casing has passed. Once the cement hardens, a smaller drill bit is used to drill through the cement in the shoe joint and further into the earth.
Guide shoes typically comprise a tapered, often bullet-nosed piece of equipment found on the bottom of a casing string. The shoe guides the casing toward the center of the hole and minimizes problems associated with hitting rock ledges or washouts in the wellbore as the casing string is lowered into the well. The outer portions of the guide shoe are typically made from steel, generally matching the casing string in size, if not steel grade. The interior is generally made of cement or thermoplastic, since this material must be drilled out if the well is to be deepened beyond its current casing point. However, the interior may also be made of only steel.
Float shoes also typically include a tapered, often bullet-nosed device fitted with a valve and are typically found at the bottom of a casing string. The float shoe prevents reverse flow, or U-tubing, of cement slurry from the annulus into the casing. The float shoe also guides the casing toward the center of the hole to minimize hitting rock ledges or washouts as the casing is run into the wellbore. The float shoe also reduces hook weight. The outer portions of the float shoe are typically made of steel and generally match the casing size, although not necessarily the casing grade. The interior is usually made of cement or thermoplastic, since this material must be drilled out if the well is to be deepened beyond its current casing point.
Guide shoes differ from float shoes in that they lack the valve that float shoes have for preventing reverse flow into the interior of the casing string. Thus, depending on the specific operation needs for a well, a well operator must specify whether to use a guide shoe or a float shoe to facilitate running the casing in the borehole. The inclusion of a valve increases the manufacturing and consumer cost of a float shoe. Therefore, some operators specify guide shoes based on cost savings if the performance is appropriate. Thus, manufacturers typically manufacture and keep both guide shoes and float shoes in inventory for servicing either type of casing installation.
An additional concern with shoes having cement interior portions is the structural integrity of the shoe during storage, transportation, and use. If the cement is subjected to enough stress, the cement may crack, chip, or break, damaging the shoe and potentially rendering the shoe useless.